Process for producing polyolefin membrane with integrally asymmetrical structure

ABSTRACT

Process for producing an integrally asymmetrical hydrophobic polyolefinic membrane with a sponge-like, open-pored, microporous support structure and a separation layer with a denser structure, using a thermally induced liquid-liquid phase separation process. A solution of at least one polyolefin is extruded to form a shaped object. The solvent used is one for which the demixing temperature of a solution of 25% by weight of the polyolefin in this solvent is 10 to 70° C. above the solidification temperature. After leaving the die, the shaped object is cooled using a liquid cooling medium that does not dissolve the polymer up to the die temperature, until the phase separation and solidification of the high-polymer-content phase take place. The integrally asymmetrical membrane producible in this manner has a porosity of greater than 30% to 75% by volume, a sponge-like, open-pored, microporous support layer without macrovoids and with on average isotropic pores, and on at least one of its surfaces a separation layer with pores &lt;100 nm, if any. The membrane is preferably used for gas separation or gas transfer processes, in particular for oxygenation of blood.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a U.S. national stage application ofInternational Application No. PCT/EP03/00084, filed on Jan. 8, 2003.

BACKGROUND OF THE INVENTION

1. Field of Invention

The invention relates to a process for producing a hydrophobic membraneusing a thermally induced phase separation process in accordance withthe preamble of Claim 1, the membrane having a sponge-like, open-pored,microporous structure, and to the use of the membrane for gas exchangeprocesses, in particular oxygenation of blood, and for gas separationprocesses.

2. Description of Related Art

In a multitude of applications in the fields of chemistry, biochemistry,or medicine, the problem arises of separating gaseous components fromliquids or adding such components to the liquids. For such gas exchangeprocesses, there is increasing use of membranes that serve as aseparation membrane between the respective liquid, from which a gaseouscomponent is to be separated or to which a gaseous component is to beadded, and a fluid that serves to absorb or release this gaseouscomponent. The fluid in this case can be either a gas or a liquidcontaining the gas component to be exchanged or capable of absorbing it.Using such membranes, a large surface can be provided for gas exchangeand, if required, direct contact between the liquid and fluid can beavoided.

Membranes are also used in many different ways to separate individualgas components from a mixture of different gases. In such membrane-basedgas separation processes, the gas mixture to be separated is directedover the surface of a membrane usable for gas separation. Sorption anddiffusion mechanisms result in a transport of the gas components throughthe membrane wall, with the transport of the individual gas componentsof the mixture occurring at different rates. This causes an enrichmentof the permeate stream passing through the membrane by the most rapidlypermeating gas component, while the retentate stream is enriched by thecomponents that permeate less readily.

This ability to separate individual gas components from a gas mixtureusing membranes finds numerous applications. For example, membrane-basedgas separation systems can be used to enrich the oxygen content of airto increase combustion efficiency or to enrich nitrogen in the air forapplications requiring an inert atmosphere.

An important application of membrane-based gas exchange processes in themedical field is for oxygenators, also called artificial lungs. In theseoxygenators, which are used in open-heart operations, for example,oxygenation of blood and removal of carbon dioxide from the blood takeplace. Generally, bundles of hollow-fiber membranes are used for suchoxygenators. Venous blood flows in this case in the exterior spacearound the hollow-fiber membranes, while air, oxygen-enriched air, oreven pure oxygen, i.e., a gas, is passed through the lumen of thehollow-fiber membranes. Via the membranes, there is contact between theblood and the gas, enabling transport of oxygen into the blood andsimultaneously transport of carbon dioxide from the blood into the gas.

In order to provide the blood with sufficient oxygen and at the sametime to remove carbon dioxide from the blood to a sufficient extent, themembranes must ensure a high degree of gas transport: a sufficientamount of oxygen must be transferred from the gas side of the membraneto the blood side and, conversely, a sufficient amount of carbon dioxidefrom the blood side of the membrane to the gas side, i.e., the gas flowor gas transfer rates, expressed as the gas volume transported per unitof time and membrane surface area from one membrane side to the other,must be high. A decisive influence on the transfer rates is exerted bythe porosity of the membrane, since only in the case of sufficientlyhigh porosity can adequate transfer rates be attained.

A number of oxygenators are in use that contain hollow-fiber membraneswith open-pored, microporous structure. One way to produce this type ofmembrane for gas exchange, such as for oxygenation, is described inDE-A-28 33 493. Using the process in accordance with this specification,membranes with up to 90% by volume of interconnected pores can beproduced from meltable thermoplastic polymers. The process is based on athermally induced phase separation process with liquid-liquid phaseseparation. In this process, a homogeneous single-phase solution isfirst prepared from the thermoplastic polymer and a compatible componentthat forms a binary system with the polymer, the system in the liquidstate of aggregation having a range of full miscibility and a range witha miscibility gap, and this solution is then extruded into a bath thatis substantially chemically inert with respect to, i.e., does notsubstantially react chemically with, the polymer and has a temperaturelower than the demixing temperature. In this way, a liquid-liquid phaseseparation is initiated and, on further cooling, the thermoplasticpolymer solidified to form the membrane structure.

The membranes in accordance with DE-A-28 33 493 have an open-pored,microporous structure and also open-pored, microporous surfaces. On theone hand, this has the result that, in gas exchange processes, gaseoussubstances such as oxygen (O₂) or carbon dioxide (CO₂) can pass throughthe membrane relatively unrestricted and the transport of a gas takesplace as a “Knudsen flow” combined with relatively high transfer ratesfor gases or high gas flow rates through the membrane. Such membraneswith gas flow rates for CO₂ exceeding 1 ml/(cm²*min*bar) and for O₂ atapproximately the same level have gas flow rates that are sufficientlyhigh for oxygenation of blood.

On the other hand, in extended-duration use of these membranes in bloodoxygenation or generally in gas exchange processes with aqueous liquids,blood plasma or a portion of the liquid can penetrate into the membraneand, in the extreme case, exit on the gas side of the membrane, even ifin these cases the membranes are produced from hydrophobic polymers, inparticular polyolefins. This results in a drastic decrease in gastransfer rates. In medical applications for blood oxygenation, this istermed plasma breakthrough.

The plasma breakthrough time of such membranes as producible inaccordance with DE-A-28 33 493 is sufficient in most cases ofconventional blood oxygenation to oxygenate a patient in a normalopen-heart operation. However, these membranes are not suitable forso-called extended-duration oxygenation due to their relatively shortplasma breakthrough times. Such membranes also cannot be used for gasseparation tasks due to their consistent open-pored structure.

However, in the field of oxygenation, the desire exists for membraneswith higher plasma breakthrough times in order to attain higher levelsof safety in extended-duration heart operations and to rule out thepossibility of a plasma breakthrough that would require immediatereplacement of the oxygenator. The aim is also to be able to oxygenatepremature infants or in general patients with temporarily restrictedlung function long enough until the lung function is restored, i.e., tobe able to conduct extended-duration oxygenation. A prerequisite forthis is appropriately long plasma breakthrough times. A frequentlydemanded minimum value for the plasma breakthrough time in thisconnection is 20 hours.

From EP-A-299 381, hollow-fiber membranes for oxygenation are known thathave plasma breakthrough times of more than 20 hours, i.e., there is noplasma breakthrough even under extended use. This is achieved with theotherwise porous membranes by using a barrier layer with an averagethickness not exceeding 2 μm and substantially impermeable to ethanol.According to the disclosed examples, the membranes in accordance withEP-A-299 381 have a porosity of at most 31% by volume, since at higherporosity values the pores are interconnected via the membrane wall andcommunication occurs between the sides of the hollow-fiber membranes,resulting in plasma breakthrough.

The production of these membranes is conducted via a melt-drawingprocess, i.e., the polymer is first melt-extruded to form a hollow fiberand then hot- and cold-drawn. In this case, only relatively low porosityvalues are obtained, which means that, in conjunction with the transportoccurring in the barrier layer via solution diffusion, the attainabletransfer rates for oxygen and carbon dioxide remain relatively low.Moreover, while the hollow-fiber membranes in accordance with EP-A-299381 exhibit sufficient tensile strength as a result of the pronounceddrawing in conjunction with manufacture, they have only a smallelongation at break. In subsequent textile processing steps, such asproducing hollow-fiber mats, which have proven excellent in theproduction of oxygenators with good exchange capacity and as aredescribed in EP-A-285 812, for example, these hollow-fiber membranes aretherefore difficult to process.

U.S. Pat. No. 4,664,681 discloses polyolefin membranes in particular forgas separation, with a microporous layer and a non-porous separationlayer, the membranes also being produced using a melt-drawing process.The properties of these membranes are similar to those described inEP-A-299 381.

Typically, in melt-drawing processes, membranes are formed withslit-shaped pores with pronounced anisotropy, the first main extensionof which is perpendicular to the drawing direction and the second mainextension perpendicular to the membrane surface, i.e., in the case ofhollow-fiber membranes runs between the exterior and interior surfacesof the membrane, so that the channels formed by the pores run in arelatively straight line between the surfaces. In the case in which, forexample, mechanical damage in the spinning process causes leaks in thebarrier layer, a preferred direction then exists for the flow of aliquid between the interior and exterior surfaces or vice versa, therebypromoting plasma breakthrough.

DE-C-27 37 745 relates to microporous bodies likewise produced using aprocess with thermally induced liquid-liquid phase separation. Duringproduction of the microporous bodies, when the polymer solution is castonto a substrate, such as a metal plate, the microporous bodiesaccording to DE-C-27 37 745 can also exhibit a surface skin with astructure not having a cellular form, the thickness of the skin being inmost cases approximately the thickness of an individual cell wall.DE-C-27 37 745, however, does not state that such microporous bodieswith a surface skin are usable for gas exchange processes, in particularextended-duration oxygenation, or for gas separation processes.Moreover, hollow-fiber membranes cannot be produced using the proceduredescribed in DE-C-27 37 745.

In WO 00/43113 and WO 00/43114, integrally asymmetrical polyolefinmembranes are disclosed, and processes for producing them described,that are usable for gas exchange, in particular extended-durationoxygenation, or also for gas separation. The processes are likewisebased on a thermally induced phase separation process with liquid-liquidphase separation. The membranes according to WO 00/43113 or WO 00/43114have a support layer with a sponge-like, open-pored, microporousstructure and, adjacent to on this support layer on at least one of thesurfaces a separation layer with a denser structure. To produce thismembrane structure, and in particular the separation layer, the citedspecifications for producing the polyolefin solutions employed startwith solvent systems consisting of a mixture of a solvent with anon-solvent for the polyolefin, where the properties of the solvent andnon-solvent must meet specific requirements. A disadvantage of theprocesses disclosed in these specifications is that solvent systems mustalways be used that are mixtures of several components. Such solventsystems are, from experience, complex with respect to the elements ofthe process that are aimed at reusing the individual components.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide a simplifiedprocess for producing integrally asymmetrical membranes with amicroporous support structure and a separation layer with a denserstructure, also in the form of hollow-fiber membranes, that are suitedfor gas exchange and at high gas exchange levels are impervious overextended periods of time to a breakthrough of hydrophilic liquids, inparticular blood plasma, or that are suited for gas separation, themembranes having good qualities for further processing.

The object is achieved by a process for producing an integrallyasymmetrical hydrophobic membrane having a sponge-like, open-pored,microporous support structure and a separation layer with a denserstructure compared to the support structure, the process comprising atleast the steps of:

a) preparing a homogeneous solution from a system comprising 20-90% byweight of a polymer component consisting of at least one polyolefin and80-10% by weight of a solvent for the polymer component, wherein thesystem at elevated temperatures has a range in which it is present as ahomogeneous solution and on cooling a critical demixing temperature,below the critical demixing temperature in the liquid state ofaggregation a miscibility gap, and a solidification temperature,

b) rendering the solution to form a shaped object, with first and secondsurfaces, in a die having a temperature above the critical demixingtemperature,

c) cooling the shaped object using a cooling medium, conditioned to acooling temperature below the solidification temperature, at such a ratethat a thermodynamic non-equilibrium liquid-liquid phase separation intoa high-polymer-content phase and a low-polymer-content phase takes placeand solidification of the high-polymer-content phase subsequently occurswhen the temperature falls below the solidification temperature,

d) possibly removing the low-polymer-content phase from the shapedobject,

characterized in that a solvent for the polymer component is selectedfor which, on cooling at a rate of 1° C./min, the demixing temperatureof a solution of 25% by weight of the polymer component in this solventis 10 to 70° C. above the solidification temperature and that, forcooling, the shaped object is brought into contact with a liquid coolingmedium that does not dissolve or react chemically with the polymercomponent at temperatures up to the die temperature.

Surprisingly, it has been shown that, by adhering to these processconditions, integrally asymmetrical membranes are obtained in which atleast one surface is formed as a separation layer that covers theadjacent sponge-like, open-pored, microporous support layer and has adenser structure compared to the support layer. The process according tothe invention allows the realization of very thin separation layers,whose structure can be adjusted from dense to nanoporous, with poreshaving an average size of less than 100 nm and in individual casesbeyond that. At the same time, the support layer of the membranesproduced in this manner has a high volume porosity.

Preferably, the process according to the invention is used to produceintegrally asymmetrical membranes with a dense separation layer. In thiscontext, a dense separation layer or dense structure is understood to beone for which no pores are evident based on an examination by scanningelectron microscope at 60000× magnification.

The process according to the invention thus permits the production ofintegrally asymmetrical membranes with a separation layer that isimpervious over long periods of time to liquid breakthrough but at thesame time gas permeable, and with a support layer with high volumeporosity, resulting at the same time in high gas transfer levels forthese membranes in gas transfer processes. These membranes findexcellent application for extended-duration blood oxygenation, theseparation layer of these membranes being responsible for making themimpervious over extended periods of time to the breakthrough of bloodplasma. At the same time, membranes with a dense separation layer can beproduced that allow high gas separation factors to be attained and canbe used for gas separation.

BRIEF DESCRIPTION OF FIGURES

FIG. 1 shows a scanning electron microscope (SEM) image of the exteriorsurface of a hollow-fiber membrane according to example 1 at 60000×magnification;

FIG. 2 shows an SEM image of the interior surface of a hollow-fibermembrane according to example 1 at 13500× magnification;

FIG. 3 shows an SEM image of the surface of fracture perpendicular tothe longitudinal axis of a hollow-fiber membrane according to example 1in the vicinity of the exterior surface at 13500× magnification;

FIG. 4 shows an SEM image of the surface of fracture perpendicular tothe longitudinal axis of a hollow-fiber membrane according to example 1in the vicinity of the interior surface at 13500× magnification;

FIG. 5 shows an SEM image of the exterior surface of a hollow-fibermembrane according to example 2 at 60000× magnification;

FIG. 6 shows an SEM image of the interior surface of a hollow-fibermembrane according to example 2 at 13500× magnification;

FIG. 7 shows an SEM image of the surface of fracture perpendicular tothe longitudinal axis of a hollow-fiber membrane according to example 2in the vicinity of the exterior surface at 13500× magnification;

FIG. 8 shows an SEM image of the exterior surface of a hollow-fibermembrane according to example 3 at 60000× magnification;

FIG. 9 shows an SEM image of the interior surface of a hollow-fibermembrane according to example 3 at 13500× magnification;

FIG. 10 shows an SEM image of the surface of fracture perpendicular tothe longitudinal axis of a hollow-fiber membrane according to example 3in the vicinity of the exterior surface at 13500× magnification;

FIG. 11 shows an SEM image of the exterior surface of a hollow-fibermembrane according to example 4 at 60000× magnification;

FIG. 12 shows an SEM image of the interior surface of a hollow-fibermembrane according to example 4 at 13500× magnification;

FIG. 13 shows an SEM image of the surface of fracture perpendicular tothe longitudinal axis of a hollow-fiber membrane according to example 4in the vicinity of the exterior surface at 13500× magnification;

FIG. 14 shows an SEM image of the exterior surface of a hollow-fibermembrane according to comparative example 1 at 60000× magnification;

FIG. 15 shows an SEM image of the interior surface of a hollow-fibermembrane according to comparative example 1 at 4500× magnification;

FIG. 16 shows an SEM image of the surface of fracture perpendicular tothe longitudinal axis of a hollow-fiber membrane according tocomparative example 1 in the vicinity of the exterior surface at 13500×magnification; and

FIG. 17 shows an SEM image of the surface of fracture perpendicular tothe longitudinal axis of a hollow-fiber membrane according tocomparative example 1 in the vicinity of the interior surface at 13500×magnification.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Within the context of the present invention, an integrally asymmetricalmembrane is understood to be one in which the separation and supportlayers consist of the same material and were formed together directlyduring membrane production, resulting in both layers being integrallyjoined with each other. In the transition from the separation layer tothe support layer, there is merely a change with respect to the membranestructure. Contrasting with this are composite membranes, for example,which have a multilayer structure formed by applying, in a separateprocess step, a dense layer as a separation layer on a porous, oftenmicroporous support layer or support membrane. The result is that thematerials constituting the support and separation layers also havedifferent properties in the case of composite membranes.

The process according to the invention is based on a thermally inducedphase separation process with liquid-liquid phase separation. Accordingto the invention, the polymer component and the solvent form a binarysystem, which in the liquid state of aggregation has a range in whichthe system is present as a homogeneous solution and a range in which itexhibits a miscibility gap in the liquid state of aggregation. If such asystem is cooled, from the range in which it is present as a homogenoussolution, below the critical demixing or phase separation temperature,liquid-liquid demixing or phase separation into two liquid phases,namely one with a high polymer content and the other with a low polymercontent, initially takes place. On further cooling, below thesolidification temperature, the high-polymer-content phase solidifies toform a three-dimensional membrane structure. The cooling rate in thiscase has a substantial influence on the pore structure being created. Ifthe cooling rate is high enough that the liquid-liquid phase separationcannot take place under thermodynamic equilibrium conditions but ratherunder thermodynamic non-equilibrium conditions and on the other handstill relatively slowly, the liquid-liquid phase separation occursapproximately concurrently with the formation of a large number ofdroplets of liquid that are of substantially the same size. Theresulting polymer object then has a sponge-like cellular and open-poredmicrostructure. If the cooling rate is significantly higher, the polymersolidifies before most of the droplets of liquid can form. In this case,sponge-like structures with network- or coral-like microstructures areformed. The variety of such sponge-like microporous structures formedvia processes with thermally induced liquid-liquid phase separation aredescribed in detail in DE-C-27 37 745, reference to the disclosure ofwhich is hereby explicitly made, and depicted for example in R. E.Kesting, “Synthetic Polymeric Membranes”, John Wiley & Sons, 1985, pp.261-264.

Generally speaking, the solvent is to be seen as a compound in which thepolymer component is completely dissolved to form a homogeneous solutionwhen heated to at most the boiling point of this compound. In thecontext of the present invention, a solvent for the at least one polymeris to be used for which, for a solution of 25% by weight of the polymercomponent in this solvent and a cooling rate of 1° C./min, the demixingtemperature is 10 to 70° C. above the solidification temperature. Suchsolvents can be categorized as weak solvents for the polymer component.A strong solvent would then be one for which, for a solution of 25% byweight of the polymer component in this solvent and a cooling rate of 1°C./min, the demixing temperature is no more than 5° C. above thesolidification temperature.

It has been observed that the use of an overly strong solvent, for whichthe difference between the demixing and solidification temperatures isless than 10° C. and which results in comparatively low solidificationtemperatures, promotes the formation of spherulitic or particle-shapedstructures and in part defective separation layers. These structures,which are outside the scope of the invention, have a structureconsisting of particle-shaped structure elements with in part rosette orlaminar construction, where the structure elements are interconnectedvia laminar or fibrillar links. The membranes produced using the citedsolvents, which are outside the scope of the invention, then do not havea sponge-like, open-pored, microporous support structure and furthermorelack sufficient mechanical stability for practical application. On theother hand, the use of overly weak solvents can result in a separationlayer that is not free of defects but rather exhibits relatively largeholes or splits.

The demixing temperature is preferably 20 to 50° C., and especiallypreferably 25 to 45° C., above the solidification temperature.

The demixing, or phase separation, temperature and the solidificationtemperature in this case can be determined in a simple manner byinitially preparing a homogeneous solution of 25% by weight of thepolymer component in the solvent under investigation and then heatingthis solution to a temperature approximately 20° C. above the dissolvingtemperature. This solution is stirred and maintained at this temperaturefor about 0.5 hours, in order to achieve sufficient homogeneity.Subsequently, the solution is cooled at a rate of 1° C./min whilestirring. The phase separation temperature is determined as thetemperature at which clouding becomes visible. On further cooling,solidification of the high-polymer-content phase begins with theappearance of individual polymer particles. The solidificationtemperature is then the temperature at which substantially all of thehigh-polymer-content phase has solidified.

The formation of spherulitic or particle-shaped structures has also beenobserved in particular when high-density polyolefins were used.Apparently, when carrying out the process according to the invention,high-density polyolefins have an increased tendency to form spheruliticor particle-shaped structures. It is presumed that the crystallizationbehavior, such as the crystallization rate, then has an increased effecton the formation of the membrane structure. Preferably, therefore, apolymer component with a density of ≦910 kg/m³ is employed.

According to the invention, the polymer component used is at least onepolyolefin. In this case, the polymer component can be a singlepolyolefin or a mixture of several polyolefins, where the polyolefinsalso include polyolefin copolymers or modified polyolefins. Mixtures ofdifferent polyolefins are interesting in that various properties such aspermeability or mechanical characteristics can be optimized thereby. Forexample, by adding just slight amounts of a polyolefin with an ultrahighmolecular weight, for example exceeding 10⁶ daltons, a strong influencecan be exerted on the mechanical properties. A prerequisite for this, ofcourse, is that the polyolefins employed in this case together besoluble in the solvent used. In the case that mixtures of severalpolyolefins are used for the polymer component, in an especiallypreferred embodiment each polyolefin contained in the mixture has adensity of <910 kg/m³.

The at least one polyolefin contained in the polymer componentpreferably consists exclusively of carbon and hydrogen. Especiallypreferred polyolefins are polypropylene and poly(4-methyl-1-pentene) ormixtures of these polyolefins among themselves. Of particular advantageis the use of poly(4-methyl-1-pentene). Particularly dense separationlayers and high gas transfer rates can be realized thereby, whilemaintaining good mechanical properties for the membranes.

For the solvent, compounds are to be used that fulfill the statedconditions. In case of the especially preferred use of polypropylene asthe polymer component, N,N-bis(2-hydroxyethyl)tallow amine, dioctylphthalate, or a mixture thereof are preferably used as solvents. In theespecially preferred use of poly(4-methyl-1-pentene) as a polyolefin,preferred solvents are palm nut oil, dibutyl phthalate, dioctylphthalate, dibenzyl ether, coconut oil, or a mixture thereof. Especiallydense separation layers are obtained using dibutyl phthalate or dibenzylether.

The fractions of polymer component and solvent required for membraneproduction can be determined by generating phase diagrams in simpleexperiments. Such phase diagrams can be developed using known methods,such as are described in C. A. Smolders, J. J. van Aartsen, A.Steenbergen, Kolloid-Z. und Z. Polymere, 243 (1971), pp. 14-20.

The polymer fraction of the system from which the solution is formed ispreferably 30-60% by weight, and the fraction of the solvent is 70-40%by weight. The polymer fraction is especially preferred to be 35-50% byweight and the fraction of the solvent 65-50% by weight. If necessary,additional substances such as antioxidants, nucleating agents, fillers,components to improve biocompatibility, i.e., blood tolerance when usingthe membrane in oxygenation, such as vitamin E, and similar substancescan be employed as additives to the polymer component, solvent, orpolymer solution.

The polymer solution formed from the polymer component and the solventis given shape using suitable dies. The shaped object preferably has theform of a film or hollow filament, and the membrane ultimately producedtherefrom is a flat or hollow-fiber membrane. Conventional dies such assheeting dies, casting molds, doctor blades, profiled dies, annular-slitdies, or hollow-filament dies can be employed.

In a preferred embodiment, hollow-fiber membranes are produced by theprocess according to the invention. In this case, the polymer solutionis extruded through the annular gap of the corresponding hollow-filamentdie to form a shaped object, i.e., a hollow filament. A fluid is meteredthrough the central bore of the hollow-filament die that acts as aninterior filler that shapes and stabilizes the lumen of the hollow-fibermembrane. The extruded hollow filament or resulting hollow-fibermembrane then exhibits a surface facing the lumen, the interior surface,and a surface facing away from the lumen, the exterior surface,separated from the interior surface by the wall of the hollow filamentor hollow-fiber membrane.

After shaping, the shaped object is cooled using the liquid coolingmedium employed in accordance with the invention, so that athermodynamic non-equilibrium liquid-liquid phase separation occurs inthe shaped object, i.e., in the shaped polymer solution, and the polymerstructure subsequently solidifies and hardens. In this process, thecooling medium has been conditioned to a temperature below thesolidification temperature. According to the invention, in order toproduce the desired integrally asymmetrical membrane with separationlayer, a liquid cooling medium is to be used that does not dissolve orreact chemically with the polymer component, even when the medium isheated to the die temperature. The use of such a cooling medium plays aprimary role in the formation of a separation layer with a denserstructure. Such a requirement placed on the cooling medium rules out,for example, the use as a cooling medium of the solvent employedaccording to the invention. Although the latter would not dissolve thepolymer component at the cooling temperature, this solvent forms ahomogeneous solution with the polymer component at the die temperature,as previously noted.

It is especially preferred for the liquid used as the cooling medium tobe a non-solvent for the polymer component, i.e., it does not dissolvethe polymer component to form a homogeneous solution when heated up tothe boiling point of the cooling medium. The liquid used as the coolingmedium can also contain a component that is a solvent for the polymercomponent, or it can also be a mixture of different non-solvents, aslong as it overall does not dissolve the polymer component attemperatures up to at least the die temperature. It is observed in thiscase that the degree of non-solvent character of the cooling mediuminfluences the tightness of the separation layer being formed. In anespecially preferred embodiment of the process according to theinvention, therefore, a liquid is used as a cooling medium that is astrong non-solvent for the polymer component. In the scope of thepresent invention, the strength of a non-solvent is assessed on thebasis of the difference between the demixing temperature of a solutionconsisting of the polymer component and a strong solvent and thedemixing temperature of a solution containing as a solvent the samesolvent and 10% by weight of the non-solvent under investigation. Thepolymer component concentration in each case is 25% by weight. A strongnon-solvent is then understood to be one that leads to an increase inthe demixing temperature of at least 10% relative to the demixingtemperature of the corresponding solution consisting of only the solventand the polymer component.

Preferably, the cooling medium at the cooling temperature is ahomogeneous, single-phase liquid. This ensures production of membraneswith especially homogeneous surface structures.

The liquid cooling medium used can be one that is miscible with thesolvent to form a homogeneous solution or one that does not dissolve thesolvent. The cooling medium is advantageously a liquid that is a strongnon-solvent for the polymer component and is homogeneously miscible withthe solvent at the cooling temperature, i.e., in which the solventdissolves at the cooling temperature.

To initiate a thermodynamic non-equilibrium liquid-liquid phaseseparation, the temperature of the cooling medium must be significantlybelow the critical demixing temperature or phase separation temperatureof the system used, consisting of the polymer component and solvent,and, in order to solidify the high-polymer-content phase, below thesolidification temperature. In this case, the formation of theseparation layer is promoted when there is as great a difference aspossible between the demixing temperature and the temperature of thecooling medium. The cooling medium preferably has a temperature at least100° C. below the phase separation temperature, and especiallypreferably a temperature that is at least 150° C. below the phaseseparation temperature. It is particularly advantageous if thetemperature of the cooling medium in this case is under 50° C. Inindividual cases, cooling to temperatures below ambient temperature canbe required. It is also possible for cooling to be performed in severalsteps.

The liquid cooling medium in which the shaped object is immersed forcooling and through which it is normally passed, can be located in atub-shaped container, for example. The liquid cooling medium ispreferably in a shaft or spinning tube which the shaped object passesthrough for cooling purposes. In this case, the cooling medium andshaped object are generally fed in the same direction through the shaftor spinning tube. The shaped object and cooling medium can be fed at thesame or different linear speeds through the spinning tube, where,depending on the requirement, either the shaped object or the coolingmedium can have the higher linear speed. Such process variants aredescribed in DE-A-28 33 493 or EP-A-133 882, for example.

The interior filler employed in extrusion of hollow filaments can be ingaseous or liquid form. When using a liquid as the interior filler, aliquid must be selected that substantially does not dissolve the polymercomponent in the shaped polymer solution below the critical demixingtemperature of the polymer solution. In other respects, the same liquidscan be used as can also be used as the cooling medium. In this manner,hollow-fiber membranes can also be produced that have a separation layeron both their outside and inside, or also hollow-fiber membranes thathave a separation layer only on their inside. Preferably, the interiorfiller is then a non-solvent for the polymer component and especiallypreferably a strong non-solvent for the polymer component. The interiorfiller in this case can be miscible with the solvent to form ahomogeneous, single-phase solution. In case the interior filler isgaseous, it can be air, a vaporous material, or preferably nitrogen orother inert gases.

It is advantageous if the exit surface of the die and the surface of thecooling medium are spatially separated by a gap, which is transited bythe shaped object prior to contact with the cooling medium. The gap canbe an air gap, or it can also be filled with another gaseous atmosphere,and it can also be heated or cooled. The polymer solution, however, canalso be brought directly into contact with the cooling medium afterexiting from the die.

In an advantageous embodiment of the process according to the invention,at least one of the surfaces of the shaped object leaving the die,preferably the surface on which the separation layer is to be formed, issubjected prior to cooling to a gaseous atmosphere promoting theevaporation of the solvent, i.e., to an atmosphere in which theevaporation of the solvent is possible. Preferably, air is used to formthe gaseous atmosphere. Likewise preferred are nitrogen or other inertgases or also vaporous media. The gaseous atmosphere is advantageouslyconditioned and generally has a temperature below that of the die. Toevaporate a sufficient fraction of the solvent, at least one of thesurfaces of the shaped object is preferably subjected to the gaseousatmosphere for at least 0.5 s. To provide the gaseous atmospherepromoting the evaporation of the solvent, it is often sufficient tospatially separate the die and cooling medium so that a gap is formedbetween them that contains the gaseous atmosphere and through which theshaped object passes.

In producing flat membranes, for example, the polymer solution extrudedthrough a sheeting die, for example, can, as a flat sheet, initially bepassed through a gap, such as an air gap, before being cooled. In thiscase, the flat sheet is enveloped on all sides, i.e., the two surfacesand the edges, by the gaseous atmosphere, influencing the formation ofthe separation layer on both surfaces of the resulting flat membrane.

In the case of producing hollow-fiber membranes, the hollow filamentleaving the die can likewise be directed through a gap formed betweenthe die and cooling medium and containing the gaseous atmosphere.

In individual cases, the structure of the separation layer can also beinfluenced by drawing the shaped polymer solution after exiting the die,i.e., particularly in the air gap, the drawing being effected byestablishing a difference between the exit speed of the polymer solutionfrom the die and the speed of the first withdrawal device for the cooledshaped object.

After cooling and hardening of the polymer structure, the solvent orlow-polymer-content phase is usually removed from the shaped object.Removal can be performed, for example, by extraction. Preferably,extraction agents are used that do not dissolve the polymer or polymersbut are miscible with the solvent. Subsequent drying at elevatedtemperatures can be necessary to remove the extraction agent from themembrane. Suitable extraction agents are acetone, methanol, ethanol, andpreferably isopropanol.

In some cases, it can also be practical to retain the solvent at leastin part in the shaped object. Other components added to the solvent asadditives can remain in the membrane structure as well and thus serve asfunctional active liquids, for example. Various examples of microporouspolymers containing functional active liquids are described in DE-C 2737 745.

Before or after the removal of at least a substantial portion of thesolvent, a slight stretching of the membrane can take place inparticular to modify the properties of the separation layer in aspecific manner. For example, in a substantially dense separation layer,stretching can be used to create pores or the size of pores in theseparation layer can be adapted to the size required by the specificapplication for the resulting membrane.

In producing membranes for extended-duration oxygenation, however, itmust be ensured that the average pore size does not exceed 100 nm, sothat premature breakthrough of liquid can be avoided. For this reason,the stretching should generally not exceed 10% when producing themembranes of the invention. The stretching can, as required, also beperformed in several directions and is advantageously performed atelevated temperatures. For example, such stretching can also beconducted during drying of the membrane that might be necessary afterextraction.

By adjusting the pore size of the separation layer, such as in adownstream stretching step, membranes for nanofiltration orultrafiltration can therefore also be produced by the process accordingto the invention.

The process according to the invention is preferably used to produce ahydrophobic integrally asymmetrical membrane, in particular for gasseparation or gas exchange, wherein the membrane is composed primarilyof at least one polyolefin, has first and second surfaces, and has anintermediate support layer with a sponge-like, open-pored, microporousstructure and adjacent to this support layer on at least one of thesurfaces a separation layer with a denser structure, where theseparation layer is dense or has pores with an average diameter <100 nm,the support layer is free of macrovoids, the pores in the support layerare on average substantially isotropic, and the membrane has a porosityin the range from greater than 30% to less than 75% by volume. For thisreason, the invention further relates to such a membrane producible bythe process according to the invention. It is especially preferable forthe membrane produced by the process according to the invention to havea dense separation layer.

The average pore diameter in the separation layer is understood to bethe mean of the diameters of the pores in the surface formed as theseparation layer, where an image of a scanning electron microscope at60000× magnification is used as a basis. In the image-analysisevaluation, the pores are assumed to have a circular cross-section. Theaverage pore diameter is the arithmetic mean of all visible pores on amembrane surface of approx. 8 μm×6 μm at 60000× magnification. In themembranes according to the invention and those produced by the processaccording to the invention, existing pores in the surface exhibiting theseparation layer are uniformly, i.e., homogeneously, distributed overthis surface.

Due to their structure, these membranes, when used for gas transfer, aredistinguished by high gas flow rates and high gas transfer rates whilemaintaining high levels of safety with respect to a breakthrough of theliquid from which a gaseous component is to be separated or to which agaseous component is to be added, and also by good mechanicalproperties. To achieve this, the membrane has a high volume porosity,where the latter is determined substantially by the structure of thesupport layer, and a defined separation layer with minimal thickness.

The support layer of the membranes produced by the process according tothe invention, or the membranes according to the invention, can, aspreviously discussed, have different structures. In one embodiment, thesupport layer has a sponge-like, cellular, and open-pored structure, inwhich the pores can be described as enveloped microcells that areinterconnected by channels, smaller pores, or passages. In anotherembodiment, the support layer has a non-cellular structure, in which thepolymer phase and the pores form interpenetrating network structures,which can also be described as coral-shaped structures. In any case,however, the support layer is free of macrovoids, i.e., free of suchpores often referred to in the literature as finger pores or caverns.

The pores of the support layer can have any geometry and be, forexample, of elongated, cylindrical, rounded shape, or also have a moreor less irregular shape. In the membranes according to the invention orthose produced by the process according to the invention, the pores inthe support layer are on average substantially isotropic. This isunderstood to mean that, although the individual pores can also have anelongated shape, the pores on average in all spatial directions havesubstantially the same extension, where deviations of up to 20% canexist between the extensions in the individual spatial directions.

With an insufficiently low volume porosity, i.e. an insufficient porefraction compared to the total volume of the membrane, the attainablegas flows and gas transfer rates are too low. On the other hand, anexcessive pore fraction in the membrane leads to deficient mechanicalproperties, and the membrane cannot be readily processed in subsequentprocessing steps. Using the process according to the invention,preferably membranes can be produced that have a volume porosity in therange of greater than 30% to less than 75% by volume and especiallypreferably greater than 50% to less than 65% by volume.

Furthermore, the membranes can have a separation layer on only one oftheir surfaces, or they can have a separation layer on both surfaces.The separation layer influences on the one hand the gas flows and gastransfer rates but on the other hand the breakthrough time, i.e., thetime the membrane is protected from a breakthrough of the liquid fromwhich, when using the membrane according to the invention, a gaseouscomponent is to be separated or to which a gaseous component is to beadded, or from a breakthrough of components contained in the liquid. Italso influences whether and how well various gases in a gas mixture canbe separated from one another, i.e., the gas separation factorα(CO₂/N₂), for example.

With a non-porous, dense separation layer, very long breakthrough timesare the result, but the transfer rates and gas flows are limited insize, since in non-porous membrane layers the gas transfer or gas flowtakes place solely via a comparatively slow solution diffusion, incontrast to the considerably greater “Knudsen flow” in porousstructures. In the case of a nanoporous separation layer, on the otherhand, the gas transfer rates and gas flows are higher than those with adense separation layer, but reduced breakthrough times can result due tothe pores.

The tightness of the separation layer and its suitability in particularfor gas separation or gas transfer can often not be evaluated withsufficient reliability solely on the basis of visual inspection, using ascanning electron microscope for example. In this case, not only thesize of existing pores or in general structural defects such as fissuresbut also their number play a role. However, the absence or presence ofpores and/or defects, as well as their number, can be evaluated byexamining the gas permeation and gas flows through the membrane as wellas the gas separation factors.

It is well known that the general principles of gas transport in polymermembranes depend on the pore size in the membrane. In membranes in whichthe separation layer has pores at most approx. 2-3 nm in size, the gaspermeates through this membrane via solution diffusion mechanisms. Thepermeability coefficient P₀ of a gas then depends solely on the polymermaterial of the membrane and on the gas itself, and the gas flow Q₀,i.e., the permeability coefficient divided by the membrane thickness,depends, for a given gas, only on the thickness of the separation layer.The gas separation factor α, which specifies the ratio of thepermeability coefficients or the gas flows Q of two gases in thismembrane, therefore depends likewise solely on the polymer material andnot, for example, on the thickness of the separation layer. For example,the gas separation factor for CO₂ and N₂ is thenα₀(CO₂/N₂)=P₀(CO₂)/P₀(N₂). For polymers in general use, resultingα₀(CO₂/N₂) values are at least 1 and generally at least 3.

In porous membranes with pores between 2 nm and about 10 μm in size, thetransport of gases takes place primarily via “Knudsen flow”. Thecalculated gas separation factors α₁, as the ratio of the measuredapparent permeability coefficients, are then inversely proportional tothe square root of the ratio of the molecular weights of the gases. Forα₁(CO₂/N₂), therefore, the result is √28/44=0.798, for example.

If a gas permeates the membranes of the present invention, which have amicroporous support structure and compared with it a denser separationlayer with pores not exceeding 100 nm on average, the permeation throughthe separation layer is the step that determines the rate. If thisseparation layer has a significant number of pores or defects, on theone hand the apparent permeability coefficients increase, but on theother hand the gas separation factor decreases. For this reason, thepresence or absence of pores and/or defects in the separation layer ofthe membranes of the invention can be determined on the basis of themeasured gas separation factors for CO₂ and N₂, α(CO₂/N₂). If the CO₂/N₂gas separation factor α(CO₂/N₂) is significantly less than 1, themembrane has an excessive number of pores or defects in the separationlayer. If the number of pores or defects in the separation layer is toohigh, however, a premature liquid breakthrough or plasma breakthroughcan no longer be ruled out with adequate certainty, and the membranesare not suitable for extended-duration use in blood oxygenation. Suchmembranes are likewise unsuitable for gas separation applications. Themembranes of the invention, therefore, preferably have a gas separationfactor α(CO₂/N₂) of at least 1, and especially preferably at least 2.

The separation layer must not be too thin, since this increases the riskof defects and thus of breakthrough, and the resulting α(CO₂/N₂) valuesare too low. However, the time to actual breakthrough is stillrelatively long in this case, since with the membranes of the inventionthere is no preferred direction for the flow of a liquid; rather, thecourse of the liquid is tortuous due to the pore structure. Contrastingwith this are membranes produced according to the aforementionedmelt-drawing process, in which, due to the pronounced anisotropy of thepores, a preferred direction for the flow of the liquids from onesurface to the other results.

While an excessively thin separation layer makes the risk of defects toogreat, an excessive separation layer thickness makes the transfer ratesand gas flow rates too low. Preferably, therefore, the thickness of theseparation layer is between 0.01 μm and 5 μm, especially preferablybetween 0.1 μm and 2 μm. Membranes of the invention with a separationlayer thickness between 0.1 μm and 0.6 μm are excellently suited. Thethickness of the separation layer can be determined for the membranes ofthe invention in a simple manner by measuring the layer using fractureimages generated by scanning electron microscopy or by ultrathin-sectioncharacterizations using transmission electron microscopy. In conjunctionwith the high porosity of the membranes, this permits the attainment ofa sufficiently high permeability of the membranes for use in bloodoxygenation and thus sufficiently high gas flows. Preferably, therefore,the membranes of the invention have a gas flow Q for CO₂, Q(CO₂), of atleast 1 ml/(cm²*min*bar).

An important application of the membranes producible by the processaccording to the invention is the oxygenation of blood. In theseapplications, as previously noted, the plasma breakthrough time plays arole, i.e., the time in which the membrane is stable against abreakthrough of blood plasma. It must be emphasized that plasmabreakthrough is a considerably more complex process than the merepenetration of a hydrophobic membrane by a hydrophilic liquid. Accordingto accepted opinion, plasma breakthrough is induced by the fact thatinitially proteins and phospholipids in the blood effect ahydrophilation of the pore system of the membrane, and in a subsequentstep a sudden penetration of blood plasma into the hydrophilated poresystem takes place. The critical variable for a liquid breakthrough istherefore considered to be the plasma breakthrough time. The membranesof the invention preferably exhibit a plasma breakthrough time of atleast 20 hours, and especially preferably a plasma breakthrough time ofat least 48 hours.

In general, in the membranes of the present invention, the transitionfrom the porous support layer to the separation layer takes place in anarrow region of the membrane wall. In a preferred embodiment, themembrane structure changes abruptly in the transition from theseparation layer to the support layer, i.e., the membrane structurechanges substantially transition-free and step-like from the microporoussupport structure to the separation layer. Membranes with such astructure have, in comparison to membranes with a gradual transitionfrom the separation layer to the support layer, the advantage of higherpermeability of the support layer for gases to be transferred, since thesupport layer is less compact in its area adjacent to the separationlayer.

In a preferred embodiment, the membranes of the invention or thoseproduced by the process according to the invention are flat membranes,which preferably have a thickness between 10 and 300 μm, especiallypreferably between 30 and 150 μm. In a likewise preferred embodiment,the membranes are hollow-fiber membranes. Depending on the embodiment,they can have a separation layer only on their interior surface, i.e. onthe surface facing the lumen, or only on their exterior surface, i.e.the surface facing away from the lumen, or on both the interior andexterior surfaces. The separation layer is preferably on the exteriorsurface. The hollow-fiber membranes preferably have an outside diameterbetween 30 and 3000 μm, especially preferably between 50 and 500 μm. Awall thickness of the hollow-fiber membrane between 5 and 150 μm isadvantageous, and a thickness between 10 and 100 μm is especiallyadvantageous. The hollow-fiber membranes have outstanding mechanicalproperties, in particular a breaking force of at least 70 cN and anelongation at break of at least 75%, readily enabling processing insubsequent textile processing steps. When using hollow-fiber membranes,it has proven beneficial for the hollow-fiber membranes, with respect tothe performance characteristics of membrane modules made therefrom, tobe initially formed, for example, by appropriate knitting processes intomats of hollow-fiber membranes substantially parallel to each other,which are then fashioned into appropriate bundles. The associatedtextile processes impose stringent demands on the mechanical propertiesof the membranes, in particular on the tensile strength and elongation.These requirements are fulfilled by the membranes of the invention andthose produced by the process according to the invention.

The membranes of the invention or those produced according to theinvention can be used in numerous applications in which a membrane isrequired with a separation layer. Preferred applications are processesfor gas separation, in which, for example, a single gas component isselectively separated from a mixture of at least two gases, or for gasenrichment, in which one or more gas components in a mixture ofdifferent gases is enriched. Furthermore, the membranes of the inventionor those produced according to the invention can be used for gastransfer processes, in which a gas dissolved in a liquid is selectivelyremoved from this liquid, and/or a gas from a mixture of gases, forexample, is dissolved in a liquid. Due to their high impermeability forplasma, i.e. to their long plasma breakthrough times, and their high gastransfer capacity for O₂ and CO₂, the membranes of the invention areexcellently suited for use in oxygenators, i.e., for the oxygenation ofblood and in particular for the extended-duration oxygenation of blood.On the other hand, in the process according to the invention, adjustmentof the pore size of the separation layer, for example in a downstreamstretching step, also preferably permits production of membranes fornanofiltration, such as for separating low-molecular substances chieflyfrom non-aqueous media, or for ultrafiltration, such as for treatingfresh water, sewage, or process water, as well as for applications inthe food, beverage, and dairy industries. The membranes of the inventionand those produced using the process of the invention can moreover alsobe used advantageously for separation or recovery of anesthesia gases,which have a considerably greater molecular diameter compared to thegases contained in respiratory air.

In the examples, the following methods were employed to characterize themembranes obtained:

Determination of the Plasma Breakthrough Time:

To determine the plasma breakthrough time, a phospholipid solutionmaintained at 37° C. (1.5 g L-α-Phosphatidy-LCholine dissolved in 500 mlphysiological saline solution) is directed with a flow of 6 l/(min*2 m²)at a pressure of 1.0 bar along one surface of a membrane sample. Air isallowed to flow along the other surface of the membrane sample, the airafter exiting the membrane sample being fed through a cooling trap. Theweight of the liquid accumulated in the cooling trap is measured as afunction of time. The time until the occurrence of a significantincrease in the weight, i.e., to the first significant accumulation ofliquid in the cooling trap, is designated as the plasma breakthroughtime.

Determination of the Volume Porosity:

A sample of at least 0.5 g of the membrane to be examined is weighed ina dry state. The membrane sample is then placed for 24 hours into aliquid that wets the membrane material but does not cause it to swell,so that the liquid penetrates into all pores. This can be detectedvisually in that the membrane sample is transformed from an opaque to aglassy, transparent state. The membrane sample is then removed from theliquid, liquid adhering to the sample removed by centrifugation at about1800 g, and the mass of the thus pretreated wet, i.e., liquid-filled,membrane sample determined.

The volume porosity in % is determined according to the followingformula:

${{Volume}\mspace{14mu}{{porosity}\mspace{11mu}\lbrack\%\rbrack}} = {100*\frac{\left( {m_{wet} - m_{dry}} \right)/\rho_{{liq}.}}{{\left( {m_{wet} - m_{dry}} \right)/\rho_{{liq}.}} + {m_{dry}/\rho_{polymer}}}}$where

m_(dry)=weight of the dry membrane sample

m_(wet)=weight of the wet, liquid-filled membrane sample

ρ_(liq.)=density of the liquid used

ρ_(polymer)=density of the membrane polymer

Determination of the Gas Flow:

To determine the gas flows, one of the sides of a membrane sample issubjected to the gas to be measured, under a constant test pressure of 2bar. In the case of hollow-fiber membranes, the gas is introduced intothe lumen of the hollow-fiber membrane for this purpose. The volumestream of the gas penetrating through the wall of the membrane sample isdetermined and standardized with respect to the test pressure and areaof the membrane sample penetrated by the gas stream. For hollow-fibermembranes, the interior surface of the membrane enclosing the lumen isemployed for this.

Determination of the Average Diameter of the Pores in the SeparationLayer:

The determination of the average diameter of the pores in the separationlayer is performed using an image-analysis technique. For this purpose,the pores are assumed to have a circular cross-section. The average porediameter is then the arithmetic mean of all visible pores on a membranesurface of approx. 8 μm×6 μm at 60000× magnification.

EXAMPLE 1

Poly(4-methyl-1-pentene) was melted stepwise in an extruder atincreasing temperatures ranging from 265° C. to 300° C. and fedcontinuously to a dynamic mixer using a gear pump. The solvent used,dibutyl phthalate (Palatinol C); was also fed, via a metering pump, tothe mixer, in which the polymer and solvent were processed together at atemperature of 290° C. to form a homogeneous solution with a polymerconcentration of 35% by weight and a solvent concentration of 65% byweight. This solution was fed to a hollow-filament die with an outsidediameter of the annular gap of 1.2 mm and extruded above the phaseseparation temperature at 240° C. to form a hollow filament. Nitrogenwas used as the interior filler. After an air section of 20 mm, thehollow filament passed through an approx. 1 m long spinning tube,through which the cooling medium, conditioned to ambient temperature,flowed. The cooling medium used was glycerin triacetate. The hollowfilament, solidified as a result of the cooling process in the spinningtube, was drawn off from the spinning tube at a rate of 72 m/min, woundonto a spool, subsequently extracted with isopropanol, and then dried at120° C.

A hollow-fiber membrane was obtained with an outside diameter of approx.415 μm, a wall thickness of approx. 90 μm, and a porosity of 57% byvolume. The outside of the membrane had an approx. 0.3 μm thickseparation layer, and the SEM examination of the exterior surface at60000× magnification indicated no pores (FIGS. 1 to 4). For the membraneaccording to this example, a CO₂ flow of 4.65 ml/(cm²*min*bar), an N₂flow of 0.54 ml/(cm²*min*bar), and thus a gas separation factorα(CO₂/N₂) of approx. 8.6 were determined. The membrane exhibited aplasma breakthrough time of more than 72 hours. After this time, themeasurement was discontinued.

EXAMPLE 2

The procedure of example 1 was followed using dibenzyl ether as thesolvent.

The hollow-fiber membrane obtained thereby had an outside diameter ofapprox. 400 μm, a wall thickness of approx. 95 μm, and a porosity ofapprox. 56% by volume. The membrane likewise had a sponge-like,microporous support structure and a 0.1 to 0.3 μm thick separation layeron its outside, and the SEM examination of the exterior surface at60000× magnification indicated no pores (FIGS. 5 to 7). For the membraneaccording to this example, on average, a CO₂ flow of 2.58ml/(cm²*min*bar), an N₂ flow of 0.83 ml/(cm²*min*bar), and a gasseparation factor α(CO₂/N₂) of 3.1 were determined. A plasmabreakthrough time of more than 72 hours was determined for the membrane.

EXAMPLE 3

The procedure of example 1 was followed using coconut oil as thesolvent. The mixer temperature was 285° C.

The resulting hollow-fiber membrane had dimensions similar to those inexample 2. On its outside, it had a thin separation layer withindividual pores up to approx. 100 nm (FIGS. 8 to 10). The CO₂ and N₂flows for the membrane of this example were on the same order ofmagnitude, from 64 to 76 ml/(cm²*min*bar).

EXAMPLE 4

The membrane was produced as for that in example 1. The solvent used,however, was palm nut oil. For cooling, a glycerin/water mixture in aratio of 65:35 was employed. The mixer temperature was set to 265° C.

The hollow-fiber membrane produced thereby had an outside diameter of406 μm and a wall thickness of 96 μm. The membrane porosity exceeded 55%by volume. The membrane had a sponge-like, microporous support structureand an approx. 0.2 μm thick separation layer on its outside. In the SEMexamination, numerous pores up to approx. 80 nm in size were observablein the exterior surface of the membrane, i.e., in the separation layer(FIGS. 11 to 13). The CO₂ and N₂ flows were 179 and 202ml/(cm²*min*bar), respectively, yielding a gas separation factorα(CO₂N₂) of 0.89.

COMPARATIVE EXAMPLE 1

The procedure of example 1 was followed. Dioctyl adipate was used as thesolvent. For dioctyl adipate, the demixing temperature of a solution of25% by weight of the poly(4-methyl-1-pentene) employed as the polymercomponent was only approx. 5° C. above the solidification temperatureand thus below the minimum level of 10° C. required by the invention.Glycerin triacetate was used as the cooling medium and was maintained atambient temperature.

The hollow-fiber membranes produced thereby had an integrallyasymmetrical structure with a dense separation layer, although theseparation layer was relatively thick at approx 3 μm. The supportstructure adjacent to the separation layer was not sponge-like butrather consisted of particle-shaped structure elements, with thestructure elements interconnected via laminar or fibrillar links (FIGS.14. to 17). Moreover, these membranes, which are outside the scope ofthe invention, had only a slight mechanical stability.

COMPARATIVE EXAMPLE 2

The membrane was produced using the process of comparative example 1.Instead of dioctyl adipate, isopropyl myristate was used as the solvent.For isopropyl myristate as well, the demixing temperature of a solutionof 25% by weight of the poly(4-methyl-1-pentene) employed as the polymercomponent was only approx. 5° C. above the solidification temperatureand thus below the minimum level of 10° C. required by the invention.

The hollow-fiber membranes produced thereby were similar to those forcomparative example 1 and had an integrally asymmetrical structure witha compact, approx. 2 μm thick separation layer. The support structureadjacent to this separation layer likewise consisted of particle-shapedstructure elements interconnected via laminar or fibrillar links.Moreover, these membranes, which are outside the scope of the invention,had only a slight mechanical stability.

1. Process for producing an integrally asymmetrical hydrophobic membraneconsisting of at least one polyolefin, the membrane having asponge-like, open-pored, microporous support structure and a separationlayer with a denser structure compared to the support structure, theprocess comprising at least the steps of: a) preparing a homogeneoussolution from a system comprising 20-90% by weight of the at least onepolyolefin and 80-10% by weight of a solvent for the at least onepolyolefin, wherein the system at elevated temperatures has a range inwhich it is present as a homogeneous solution, on cooling has a criticaldemixing temperature, below the critical demixing temperature in theliquid state of aggregation has a miscibility gap, and has asolidification temperature, b) rendering the solution to form a shapedobject, with first and second surfaces, in a die at a die temperatureabove the critical demixing temperature, c) cooling the shaped object bycontacting the shaped object with a liquid cooling medium that does notdissolve or react chemically with the at least one polyolefin attemperatures up to the die temperature, the liquid cooling medium beingconditioned to a cooling temperature below the solidificationtemperature, at such a rate that a thermodynamic non-equilibriumliquid-liquid phase separation into a high-polymer-content phase and alow-polymer-content phase takes place and solidification of thehigh-polymer-content phase subsequently occurs when the temperature ofthe shaped object falls below the solidification temperature, and d)optionally removing the low-polymer-content phase from the shapedobject, wherein a solvent for the at least one polyolefin is selectedfor which, on cooling at a rate of 1° C./min, the demixing temperatureof a solution of 25% by weight of the at least one polyolefin in thesolvent is 10 to 70° C. above the solidification temperature.
 2. Processfor producing a membrane according to claim 1, wherein the solvent forthe at least one polyolefin is one for which, for a solution of 25% byweight of the at least one polyolefin in the solvent and a cooling rateof 1° C./min, the critical demixing temperature is 20 to 50° C. abovethe solidification temperature.
 3. Process for producing a membraneaccording to claim 1, wherein the solvent for the at least one polymeris one for which, for a solution of 25% by weight of the at least onepolyolefin in the solvent and a cooling rate of 1° C./min, the criticaldemixing temperature is 25 to 45° C. above the solidificationtemperature.
 4. Process for producing a membrane according to claim 1,wherein the at least one polyolefin has a density of ≦910 kg/m³. 5.Process for producing a membrane according to claim 1, wherein theliquid cooling medium is a non-solvent for the at least one polyolefinthat, on heating up to a boiling point of the non-solvent, does notdissolve the at least one polyolefin to form a homogeneous solution. 6.Process for producing a membrane according to claim 1, wherein theliquid cooling medium is a liquid that is a strong non-solvent for theat least one polyolefin and is homogeneously miscible with the solventat the cooling temperature.
 7. Process for producing a membraneaccording to claim 1, wherein the liquid cooling medium has atemperature that is at least 100° C. below the critical demixingtemperature.
 8. Process for producing a membrane according to claim 1,wherein 30-60% by weight of the at least one polyolefin is dissolved in70-40% by weight of the solvent.
 9. Process for producing a membraneaccording to claim 1, wherein the at least one polyolefin consistsexclusively of carbon and hydrogen.
 10. Process for producing a membraneaccording to claim 9, wherein the at least one polyolefin is apoly(4-methyl-1-pentene).
 11. Process for producing a membrane accordingto claim 9, wherein the at least one polyolefin is a polypropylene. 12.Process for producing a membrane according to claim 9, wherein the atleast one polyolefin is a mixture of a poly(4-methyl-1-pentene) and apolypropylene.
 13. Process for producing a membrane according to claim10, wherein the solvent is palm nut oil, dibutyl phthalate, dioctylphthalate, dibenzyl ether, coconut oil, or a mixture thereof. 14.Process for producing a membrane according to claim 11, wherein thesolvent is N,N-bis(2-hydroxyethyl)tallow amine, dioctyl phthalate, or amixture thereof.
 15. Process for producing a membrane according to claim1, wherein the membrane is a hollow-fiber membrane.